A fuel cell converts chemical energy into electrical energy and some thermal energy by means of a chemical reaction between a fuel (e.g., hydrogen gas or a hydrogen-containing fluid) and an oxidant (e.g., oxygen). A proton exchange membrane (PEM) fuel cell uses hydrogen or hydrogen-rich reformed gases as the fuel, a direct-methanol fuel cell (DMFC) uses methanol-water solution as the fuel, and a direct ethanol fuel cell (DEFC) uses ethanol-water solution as the fuel, etc. These types of fuel cells that require utilization of a PEM layer as a proton transport electrolyte are collectively referred to as PEM-type fuel cells.
A PEM-type fuel cell is typically composed of a seven-layered structure, including (a) a central PEM electrolyte layer for proton transport; (b) two electro-catalyst layers on the two opposite primary surfaces of the electrolyte membrane; (c) two fuel or gas diffusion electrodes (GDEs, hereinafter also referred to as diffusers) or backing layers stacked on the corresponding electro-catalyst layers (each GDE comprising porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell); and (d) two flow field plates (or a bi-polar plate) stacked on the GDEs. The flow field plates are typically made of graphite, metal, or conducting composite materials, which also serve as electron transport paths. Gas-guiding channels are defined on a GDE facing a flow field plate or, more typically, on a flow field plate surface facing a GDE. Reactants (e.g., H2 or methanol solution) and reaction products (e.g., CO2 at the anode of a DMFC, and water at the cathode side) are guided to flow into or out of the cell through the flow field plates. The configuration mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units that are electrically connected in series to provide a desired output voltage. If desired, cooling channels and humidifying plates may be added to assist in the operation of a fuel cell stack.
In one common practice, a fuel flow field plate and an oxidant gas flow field plate are separately made and then assembled together to form a bipolar plate (one side of a bipolar plate serving as a negative terminal and the other side as a positive terminal, hence the name). In some cases, an additional separator is sandwiched between the two flow field plates to form a bipolar plate. It would be highly advantageous if the flow filed plates and the separator can be mass-produced into an integrated bipolar plate assembly. This could significantly reduce the overall fuel cell production costs and reduce contact ohmic losses across constituent plate interfaces. The bipolar plate is known to significantly impact the performance, durability, and cost of a fuel cell system. The bipolar plate, which is typically machined from graphite, is one of the most costly components in a PEM fuel cell.
Fluid flow field plates or bipolar plates have open-faced channels formed in one or both opposing major surfaces for distributing reactants to the gas diffuser plates, which are the anode and cathode backing layers, typically made of carbon paper or fabric. The open-faced channels also provide passages for the removal of reaction products and depleted reactant streams. Optionally, a bipolar plate may have coolant channels to manage the fuel cell temperature. According to the US Department of Energy (DOE), a bipolar plate should have the following desirable characteristics: high electrical conductivity (e.g., preferably having a conductivity no less than 100 S/cm and specific areal conductivity no less than 200 S/cm2), low permeability to fuel (e.g., hydrogen) or oxidant fluids (e.g., air or oxygen), good corrosion resistance, and good structural integrity. The specific areal conductivity is essentially the bipolar plate thickness-direction conductivity divided by the plate thickness. Hence, it is highly desirable to have a thinner plate. Current graphite bipolar plates, typically 3-5 mm thick, should preferably be reduced to below 1 mm and most preferably below 0.5 mm.
Conventional methods of fabricating fluid flow field plates require the engraving or milling of flow channels into the surface of rigid plates formed of a metal, graphite, or carbon-resin composite. Such plates are expensive due to high machining costs. The machining of channels into the graphite plate surfaces causes significant tool wear and requires significant processing times. Metals can be readily shaped into very thin plates, but long-term corrosion is a major concern. A corrosion-resistant coating may be used, but it has to be applied perfectly. The coating may also increase contact resistance.
Alternatively, fluid flow field plates can be made by a lamination process (e.g., U.S. Pat. No. 5,300,370, issued Apr. 5, 1994), wherein an electrically conductive, fluid impermeable separator layer and an electrically conductive stencil layer are consolidated to form one open-faced channel. Presumably, two conductive stencil layers and one separator layer may be laminated to form a bipolar plate. It is often difficult and time-consuming to properly position and align the separator and stencil layers. Die-cutting of stencil layers require a minimum layer thickness, which limits the extent to which fuel cell stack thickness can be reduced. Such laminated fluid flow field assemblies tend to have higher manufacturing costs than integrated plates, due to the number of manufacturing steps associated with forming and consolidating the separate layers. Corrosion also presents a challenging issue for metal stencil- or separator-based bipolar plates in a PEM fuel cell since they are used in an acidic environment.
A variety of composite bipolar plates have been developed, which are mostly made by compression molding of polymer matrices (thermoplastic or thermoset resins) filled with conductive particles such as graphite powders or fibers. Because most polymers have extremely low electronic conductivity, excessive conductive fillers have to be incorporated, resulting in an extremely high viscosity of the filled polymer melt or liquid resin and, hence, making it very difficult to process. Bi-polar plates for use in PEM fuel cells constructed of graphite powder/fiber filled resin composite materials and having gas flow channels are reviewed by Wilson, et al (U.S. Pat. No. 6,248,467, Jun. 19, 2001). Injection-molded composite-based bipolar plates are disclosed by Saito, et al. (U.S. Pat. No. 6,881,512, Apr. 19, 2005 and U.S. Pat. No. 6,939,638, Sep. 6, 2005). These thermoplastic or thermoset composites exhibit a bulk conductivity significantly lower than 100 S/cm (the US Department of Energy target value), typically not much higher than 10 S/cm.
Besmann, et al. disclosed a carbon/carbon composite-based bipolar plate (U.S. Pat. No. 6,171,720 (Jan. 9, 2001) and No. 6,037,073 (Mar. 14, 2000)). The manufacture process consists of multiple steps, including production of a carbon fiber/phenolic resin preform via slurry molding, followed by a compression-molding step. The molded part is then pyrolyzed at a high temperature (1,500° C.-2,500° C.) to obtain a highly porous carbon/carbon composite. This is followed by chemical vapor infiltration (CVI) of a carbon matrix into this porous structure. It is well-known that CVI is a very time-consuming and energy-intensive process and the resulting carbon/carbon composite, although exhibiting a high electrical conductivity, is very expensive.
Instead of using pyrolyzation and CVI to produce carbon/carbon composites, Huang, et al. (US Patent Application Pub. No. 2004/0229993, Nov. 18, 2004) discloses a process to produce a thermoplastic composite with a high graphite loading. First, polymer fibers, such as thermotropic liquid crystalline polymers or polyester, reinforcing fibers such as glass fibers, and graphite particles are combined with water to form a slurry. The slurry is pumped and deposited onto a sieve screen. The sieve screen serves the function of separating the water from the mixture of polymer fibers, glass fibers and graphite. The mixture forms a wet-lay sheet which is placed in an oven. Upon heating to a temperature sufficient to melt the polymer fibers, the wet-lay sheet is allowed to cool and have the polymer material solidify. Upon solidification, the wet-lay sheet takes the form of a sheet material with reinforcement glass fibers held together by globules of thermoplastic material, and graphite particles adhered to the sheet material by the thermoplastic material. Several of these sheets are then stacked, preferably with additional graphite powder interspersed between sheets, and compression-molded in a hot press. After application of heat and pressure in the press, one or more formed bipolar plates are obtained, where the bipolar plates are a composite of glass fibers, thermoplastic matrix and graphite particles. Clearly, this is also a tedious process which is not amenable to mass production.
Alternatively, fluid flow field plates can be made from an electrically conductive material that is sufficiently compressible or moldable so as to permit embossing. Flexible graphite sheet is generally suitable for this purpose because it is compressible and embossing processes may be used to form channels in one or both major surfaces of a sheet. The “flexible graphite” is typically obtained by first treating natural graphite particles with an agent that intercalates into the crystal structure of the graphite to form a graphite intercalated compound (GIC). The GIC is then exposed to a thermal shock up to a temperature of typically 800-1,100° C.) to expand the intercalated particles by typically 80-300 times in the direction perpendicular to the carbon layers in the crystal structure. The resulting expanded or exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. Hereinafter, the term “exfoliated graphite” will be used interchangeably with the term “expanded graphite.” The worms may be re-compressed together into flexible sheets which can be formed and cut into various shapes. These thin sheets (foils or films) are commonly referred to as flexible graphite. Flexible graphite can be wound up on a drum to form a roll of thin film, just like a roll of thin plastic film or paper.
Although a flexible graphite sheet is highly conductive in the directions parallel to the two opposed surfaces of a sheet (in-plane conductivity typically in the range of 1,100-1,750 S/cm), the thickness-direction is typically rather poor (typically 3-15 S/cm). Furthermore, flexible graphite sheets by themselves do not have sufficient stiffness and must be supported by a core layer or impregnated with a resin. For example, U.S. Pat. No. 5,527,363 (Jun. 18, 1996) discloses a fluid flow field plate comprising a metal sheet interposed between two flexible graphite (FG) sheets having flow channels embossed on a major surface thereof. These FG-metal-FG laminates are subject to the delamination or blistering problem, which could weaken the plate and may make it more fluid permeable. Delamination or blistering can also cause surface defects that may affect the flow channels on the plate. These problems may be difficult to detect during fabrication and may only emerge at a later date. The vastly different coefficients of thermal expansion (CTE) and elastic constants between a metal and a flexible graphite layer result in many challenging problems. In particular, thermal cycling between frozen and thawed states, as are likely to be encountered in an automobile application of the fuel cell, could result in delamination between a flexible graphite sheet and the metal layer if they are not adequately bonded together.
Flexible graphite (FG) sheets adequately bonded by a nanocomposite layer were disclosed in two of our earlier patent applications: (1) Bor Z. Jang, “Sheet Molding Compound Flow Field Plate, Bipolar Plate and Fuel Cell,” U.S. patent Ser. No. 11/293,540 (Dec. 5, 2005) and (2) Bor Z. Jang, A. Zhamu, and Lulu Song, “Method for Producing Highly Conductive Sheet Molding Compound, Fuel cell Flow Field Plate, and Bipolar Plate,” U.S. patent Ser. No. 11/293,541 (Dec. 5, 2005). These earlier applications were related to a composition that has three layers: a top conductive FG sheet, a middle conductive filler-resin mixture layer, and a bottom conductive FG sheet. This three-layer structure, after embossing or molding, becomes a flexible graphite-based sheet molding compound (FG-SMC).
Alternatively, Mercuri, et al. (e.g., U.S. Pat. No. 5,885,728, Mar. 23, 1999 and U.S. Pat. No. 6,037,074, Mar. 14, 2000) disclosed a flexible graphite sheet having embedded ceramic or glass fibers extending from its surface into the sheet to increase the resin permeability of the sheet for the preparation of a resin-impregnated flexible graphite bipolar plate. By allowing ceramic or glass fibers to puncture through layers of exfoliated graphite one also leaves these layers vulnerable to gas permeation, thereby significantly reducing the hydrogen and oxygen permeation resistance of a bipolar plate and increasing the chance of dangerous mixing of hydrogen and oxygen inside a fuel cell stack.
What follows is a summary of the state of the art of the resin-impregnated expanded graphite composite, resin-impregnated flexible graphite sheet composite, and methods of producing these composites: (It may be noted that these composites were not developed with fuel cell bipolar plates in mind (except for those by Mercuri, et al). The gas permeation resistance of these composites was not considered. These composites were not multi-functional laminates comprising at least two layers with at least one layer providing permeation resistance and at least another layer providing corrosion resistance or bonding strength to prevent delamination).
Olstowski, et al. (“Novel Compressed Cohered Graphite Structures and Method of Preparing Same,” U.S. Pat. No. 3,492,197, Jan. 27, 1970) provided compressed and resin-bonded forms of expanded vermicular graphite. The resin-bonded composite is obtained by (a) providing a supply of an expanded vermicular graphite having an apparent bulk density of 0.2-2.0 pounds per cubic foot; (b) providing a supply of a bonding agent; (c) blending the expanded vermicular graphite and bonding agent in an amount of 2-35 weight percent bonding agent based on the total weight of the expanded graphite-bonding agent mixture; (d) compressing the mixture at a pressure of 5-50,000 psi in predetermined directions into predetermined forms of cohered graphite; and (e) treating the so-formed composite to activate the bonding agent thereby promoting adhesion within the compact. This invention taught about compressing vermicular-bonding agent mixture in a uniaxial direction to produce a highly anisotropic composite and in bi-axial, tri-axial, cylinder-radial, and isostatic directions to produce less anisotropic or more isotropic composites. However, it failed to teach, implicitly or explicitly, how a desired degree of isotropy could be maintained when the bi-axially, tri-axially, cylinder-radially, and isostatically compressed composite compacts (prior to curing or fusing to consolidate) were re-compressed or molded as a final operation to become a thin composite plate. This thin plate (thinner than 5 mm, preferably thinner than 3 mm, further preferably thinner than 1 mm, and most preferably thinner than 0.5 mm) is for a bipolar plate application. Further, this patent was limited to using a solid bonding agent to begin with the blending process, excluding liquid polymers from the invention due to the perceived notion that these liquid polymers “can prevent formation of highly densified composites.” This patent did not teach how bi-axial, tri-axial, cylinder-radial, and isostatic compressions could be accomplished in a real manufacturing environment for the mass production of less anisotropic composites. Furthermore, the method disclosed in this patent entailed first exfoliating graphite to obtain graphite worms and then mixing graphite worms with a bonding agent in a fine solid powder form. Once the graphite worms are formed, it would be very difficult to mix the worms with fine solid particles in a homogeneous manner without breaking up or significantly disturbing the continuous network of electron-transport paths (interconnected graphite flakes).
Caines (“Vermicular Expanded Graphite Composite Materials,” U.S. Pat. No. 4,265,952, May 5, 1981) disclosed an expanded graphite composite containing a corrosion resistant resin (e.g., polytetrafluoroethylene, PTFE). The composite was prepared by blending vermicular graphite with a suspension of fine solid resin particles in a carrier liquid medium, vaporizing the carrier, and heating the composite material to sinter the resin. No electrical property of the resulting composite was reported.
Atkinson, et al. (“Housing for Electrical or Electronic Equipment,” U.S. Pat. No. 4,530,949, Jul. 23, 1985) provided a low-density composite composition consisting of exfoliated graphite and a thermosetting resin binder. The density (<0.1 gm/cm3) and the electrical conductivity (0.1 S/cm) values are relatively low.
Fukuda, et al. (“Reinforced Flexible Graphite Sheet,” U.S. Pat. No. 4,729,910, Mar. 8, 1988) disclosed a process of producing thermosetting resin reinforced flexible graphite sheets. The process involved subjecting both the flexible graphite sheet and a phenolic resin solution to a preliminary de-aeration treatment prior to immersing the flexible graphite sheet in the resin solution. No electrical conductivity data was offered.
Chung provided a low-density (0.7 gm/cm3) exfoliated flexible graphite flake-reinforced composite with a conductivity of 2 S/cm (Chung, “Low-Density Graphite-Polymer Electrical Conductor,” U.S. Pat. No. 4,704,231, Nov. 3, 1987). Chung also provided an in-situ exfoliation method of producing graphite flake-reinforced epoxy composites (“Composites of 1n-Situ Exfoliated Graphite,” U.S. Pat. No. 4,946,892, Aug. 7, 1990).
Fong, et al. (“Methacrylate Impregnated Carbonaceous Parts,” U.S. patent application Ser. No. 09/896,178, filed on Jun. 29, 2001 (Pub. No. US 2001/0046560, Pub date Nov. 29, 2001)) disclosed a method of impregnating a highly porous carbon material with a methacrylate polymer. No electrical conductivity data was provided.
Öttinger, et al. (“Impregnated Bodies Made of Expanded Graphite, Process for Producing Such Bodies and Sealing Elements, Fuel Cell Components and Heat-Conducting Elements Formed of the Bodies,” U.S. Pat. No. 6,746,771, Jun. 8, 2004) provided composites of expanded graphite impregnated with isocyanate or epoxy resins. The method involved soaking expanded graphite with a low-viscosity, polymerizing resin. The achievable electrical conductivity of the resulting composites appears to be in the range of 2-10 S/cm.
Da Silva, et al. (“Method for Producing Composite Objects Using Expanded Graphite and Vermiculite,” U.S. patent application Ser. No. 10/574,803 filed on Oct. 8, 2004 (Pub. No. US 2007/0015267, Pub date Jan. 18, 2007)) disclosed a method of producing s composite object consisting of at least two distinct parts.
Mercuri, et al. (“Flexible Graphite Article and Method of Manufacture,” U.S. Pat. No. 6,432,336, Aug. 13, 2002 and No. 6,706,400, Mar. 16, 2004) disclosed a resin-impregnated flexible graphite sheet exhibiting enhanced isotropy and a method of producing resin-impregnated flexible graphite sheet. The method includes the steps of (i) reacting raw natural graphite flake-like particles with a liquid intercalant solution to form intercalated graphite particles; (ii) exposing the intercalated graphite particles to a temperature of at least about 700° C. to expand the intercalated graphite particles to form a stream of exfoliated graphite particles; (iii) continuously compressing the stream of exfoliated graphite particles into a continuous coherent self-supporting mat of flexible graphite; (iv) continuously contacting the flexible graphite mat with liquid resin and impregnating the mat with liquid resin; and (v) continuously calendering the flexible graphite mat to increase the density thereof to form a continuous flexible graphite sheet having a thickness of no more than about 1.0 inch.
It is of interest to note that this process disclosed by Mercuri, et al. involves compressing the exfoliated graphite into a flat mat prior to impregnating the mat with a resin. This sequence is disadvantageous in that the re-compressed flexible graphite, being much denser, is less permeable to resin impregnation. Furthermore, uniaxial re-compression of the exfoliated graphite prior to resin impregnation tends to align or orientate the graphite flakes along the graphite sheet plane direction (perpendicular to the re-compression vector), resulting in a more anisotropic flexible graphite sheet composite. Once these flakes were well-aligned in a sheet to form a highly cohered mat, their orientations could no longer be changed during subsequent resin impregnation and molding operations. Furthermore, no attempt was made to re-compress the mat in different directions. Thin graphite flakes are essentially single crystals with the flake plane parallel to the basal plane and, hence, exhibit a high electrical conductivity along thin flake plane directions and much lower conductivity along the thickness direction, or c-axis direction. Consequently, the bipolar plates prepared by using the Mercuri process are not expected to have a high thickness-direction conductivity.
The resin-impregnated flexible graphite sheet exhibiting enhanced isotropy as disclosed by Mercuri, et al. (U.S. Pat. No. 6,706,400) was said to contain interlocked particles of expanded graphite. A portion of these interlocked particles of expanded graphite was substantially unaligned with the opposed planar surfaces. However, Mercuri, et al. did not fairly specify how these unaligned graphite flakes were obtained. Presumably, this could be achieved by mixing large particles of exfoliated graphite with smaller particles of exfoliated graphite, as implied in a Mercuri's earlier patent (U.S. Pat. No. 5,846,459, Dec. 8, 1998). The trade literature published by GrafTech (assignee of Mercuri's patents) indicates the electrical resistivity of bipolar plates in the X-Y plane as 7 μOhm-m (in-plane conductivity=1428 S/cm) and in the Z-direction as 300 μOhm-m (i.e., thickness-direction conductivity=33 S/cm). The thickness-direction conductivity is unsatisfactory.
In addition to exhibiting high electrical conductivity, good resistance to gas permeation, and corrosion resistance, the flow field plate or bipolar plate should be constructed from inexpensive starting materials, materials that are easily formed into any plate configuration, preferably using a continuous molding process, and materials that do not require further processing such as high temperature pyrolyzation treatments. The above review clearly indicates that prior art bipolar plate material compositions and processes have not provided a satisfactory solution for the fuel cell industry.
These issues were addressed in two of our co-pending US patent applications: Aruna Zhamu, Jinjun Shi, Jiusheng Guo, and Bor Z. Jang, “Exfoliated Graphite Composite Compositions for Fuel Cell Flow Field Plates,” Ser. No. 11/800,729 (May 8, 2007) and Aruna Zhamu, Jinjun Shi, Jiusheng Guo, and Bor Z. Jang, “Method of Producing Exfoliated Graphite Composite Compositions for Fuel Cell Flow Field Plates,” Ser. No. 11/800,730 (May 8, 2007). These two applications provide an electrically conductive, less anisotropic, and structurally sound composite composition for fuel cell flow field plate or bipolar plate applications and methods of producing this composite composition. The composite composition comprises: (a) expanded or exfoliated graphite; (b) particles of non-expandable graphite or carbon, wherein these particles are between 3% and 60% by weight based on the total weight of the particles and the expanded graphite; and (c) a binder or matrix material to bond the expanded graphite and the particles of non-expanded graphite or carbon for forming a highly conductive composite, wherein the binder or matrix material is between 3% and 60% by weight based on the total composite composition weight. The composite plate exhibits a thickness-direction conductivity typically greater than 35 S/cm, more typically greater than 50 S/cm, most typically greater than 100 S/cm, and a thickness-direction specific areal conductivity greater than 200 S/cm2, more typically greater than 500-1,500 S/cm2. These two applications did not address the issue of hydrogen gas permeation resistance of the resulting composite. In a preferred embodiment of the instant application, a thin metal layer is sandwich between two exfoliated graphite composite sheets to form a three-layer laminate. The top and bottom composite sheets are of identical or similar compositions to those disclosed in these two previous applications, but a core metal sheet is incorporated for the primary purpose of imparting exceptional hydrogen permeation resistance.
Accordingly, an object of the present invention is to provide a multi-layer laminate composition comprising at least an exfoliated graphite composite sheet and a thin metal sheet wherein both the composite sheet and the metal sheet exhibit a relatively high thickness-direction conductivity and wherein the laminate has a high resistance to gas permeation.
Another object of the present invention is to provide a laminate composition that can be easily molded or embossed into a flow field plate or bipolar plate having surface flow field channels and other desired geometric features.
Still another object of the present invention is to provide a laminate that comprises two highly conducting exfoliated graphite composite sheets sandwiching a metal sheet or foil.
Yet another object of the present invention is to provide a process for producing a laminate comprising exfoliated graphite composite sheets. Such a process can be continuous, automated, and adaptable for mass production of bipolar plates.